When a weft thread is being transported in the aforedescribed manner through the shed of a shuttleless loom, it is to be held taut especially at the instants of engagement of and disengagement from a rapier. For this purpose, it is already known to use weft brakes disposed laterally of a loom which grip the oncoming thread between two jaws and release it during its entrainment by the rapier. Usually one of the jaws is stationary while the other is movable and urged under a spring force against the first jaw; this second, movable jaw is intermittently deflected by a control pin, synchronized with the warp-engaging heddles and with the weft-transport mechanism, to separate it from the first jaw.
A drawback of the known weft brake is that the thread is completely uncontrolled upon being unclamped. The separation of the jaws, moreover, enables the entry of accompanying lint into the brake where particles thereof may accumulate and interfere with its operation. Furthermore, the impact of the sudden release of the movable jaw from its unclamping position may be harmful to the thread if the jaw is strongly spring-loaded as is required for firm clamping.